Just in case a picture helps...

If you can see how your tree diagram relates to this one...

(with P = testing positive for the Down's, N = testing negative, D = Down's present, A = Down's absent)

... then you might be able to see how I've rearranged it again here...

... and how to use this.

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Don't integrate - balloontegrate!

Balloon Calculus; standard integrals, derivatives and methods

Balloon Calculus Drawing with LaTeX and Asymptote!